In the past years, significant efforts have been devoted to develop processes for the production of pulp and paper products aimed at reducing manufacturing costs while improving product quality. Quality control of the raw materials entering in the production of pulp and paper products using either chemical or thermo mechanical pulping (TMP) processes, particularly regarding wood chips used, has been identified as a key factor in process optimization. The bleaching agent (ex. hydrosulphite, peroxide) is widely used to improve pulp brightness to a specified level which fulfills customers needs. Especially for TMP, the objective of process control is to have stable, high pulp quality with minimal energy and bleaching agent consumption. Thus, a constant flow of chips into the refiners is an important parameter for maintaining good refining performance. As a raw material, wood chips are heterogeneous and when fed into a refiner can vary in terms of wood chip species, chip size distribution, moisture content, bulk and basic density, freshness and impurities content (bark, knot, rot, etc.). These variations disturb in turn TMP process control and influence pulp quality, as mentioned by Smook, G. A. in “Handbook for Pulp & Paper Technologies”, Joint Texbook Committee of the Paper Industry, 54, (1982), as well as by Wood, J. R. in “Chip Quality Effects in Mechanical Pulping—a Selected Review”, TAPPI Pulping Conference, Proceedings, 491-497 (1996). More specifically, the variations in the wood raw materials and its fibers may give rise to 30-40% of variations in the pulp properties as taught by Lundgwist, S. O., et al, in “Wood Fiber Simulation—A Model Based Tool for Optimized Wood and Fiber Utilization”, Control System 2002, Proceedings, June 3-5, Stockholm, Sweden, 164-169 (2002).
In mills, visual evaluation of wood chip quality is widely used. From the chip color, a specialist can determine the chip species and estimate freshness, bark, rot, and knot contents. A known approach consists of sorting trees according to their species or blend of species prior to wood chips manufacturing, to produce corresponding batches of wood chips presenting desired characteristics associated with these species. Typically, hardwood trees such as poplar, birch and maple are known to generally produce pale wood chips while conifers such as pine, fir and spruce are known to generally yield darker wood chips. In practice, wood chips batches can either be produced from trees of a same species or from a blend of wood chips made from trees of plural species, preferably of a common category, i.e., hardwood trees or conifers, to seek wood chips uniformity. However, chips characteristics basically depending on initial bark content of wood chips used, the mere knowledge of wood chips species composition for a given batch does not necessarily give a reliable indication of the chips quality.
Many studies have shown that wood species is the dominant factor in pulping performance and pulp quality. The spruce family is the most favourable species for TMP as mentioned by Varhimo, A. et al, in “Raw Materials” in Sundbolm, J. “Mechanical Pulping” Chapter 5, Fapet OY, 66-104 (1999). Homogeneity and low fines content of chip size distribution produce good pulp strength, while knot and bark contents decrease the strength and brightness of the pulp as mentioned by Brill, J. W. in “Effects of Wood and Chip Quality on TMP Properties”. International Mechanical Pulping Conference, Proceedings, Stockholm 153-162 (1985). Continuous variations in wood basic density and moisture content occurring in chip flow tend to cause variations in refining consistency, which in turn affect pulp uniformity and energy consumption as mentioned by Trväinen, J. in “The influence of Wood Properties on the Quality of TMP Made From Norway Spruce (Picea abies)—Wood From Old growth Forests, First-thinning, and Sawmill Chips,” International Mechanical Pulping Conference, Proceedings, 23-34 (1995). Rot should be avoided as it impairs the brightness and strength properties of paper as discussed by Hartler, N. in “Wood Quality Requirements in Mechanical Pulping”, Nordic Pulp and Paper Research Journal. (1): 4 (1986). Knots produce low strength pulp and are predominant among oversize chips. They also reduce pulp brightness as mentioned by Brill and Wood in the above cited papers. Fresh wood chips increase productivity, decrease hydrosulfite consumption, and stabilize the pulping process as mentioned by Ding, F. et al, in “Economizing the Bleaching Agent Consumption by Controlling Wood Chip Brightness”, Control System 2002, Proceedings, June 3-5, Stockholm, Sweden, 205-209 (2002). A quantitative evaluation of pulpwood chip quality has been proposed for the chemical pulping process by Hatton, J. V. in “Chip Quality Monograph”, Joint Textbook Committee of the Paper Industry. 311 (1999). However, this evaluation being wholly based on offline laboratory measurements, accuracy depends on sampling method, frequency, and quantities. Because offline measurements cannot be used to stabilize, predict, and optimize processes, this evaluation can seldom be used in the industry. An online approach to control quality of wood chips employed in the wood chips manufacturing method disclosed in U.S. Pat. No. 5,577,671 issued on Nov. 26, 1996 to Seppanen, which method consists of separating from ground whole-tree chips, bark and cellulose wood chips through a series of separation stages including pneumatic separation, vibration segregation with sieve and color difference sorting. The resulting low bark, pale wood chips can be then processed using a minimum quantity of bleaching agent. Although processing cost can be minimized accordingly, added manufacturing cost due to bark separation step may still maintain overall production cost at a high level.
An improved online technology is discussed in U.S. Pat. No. 6,398,914 B1 issued to the present assignee, which discloses a method and apparatus for classifying batches of wood chips according to light reflection characteristics to allow optimal use of dark wood chips in pulp an paper processes. Chip brightness is a characteristic related to chip freshness, a very important parameter for TMP process, which presents the chip aging state. Chip aging is a very complex phenomenon that depends on the wood species, log and chip storage, and ambient air condition. It is very difficult and unnecessary to estimate effective chip age from its actual aging state. Although chip aging can be observed from chip brightness, it is only useful for substantially unvaried wood species. When an unknown proportion of wood species is present, more information is needed to provide reliable chip quality assessment.
The importance of chip quality for pulp and paper production processes, and especially for the TMP process has long been recognized, but heretofore, no multi-variable, reliable method for estimating the quality of wood chips has been proposed using known offline or online measurement technologies.